This invention relates to membranes useful to separate solutes or dispersed materials having relatively low molecular weights from higher molecular weight materials.
Porous membranes for separation of minute particles, typically one to ten microns in diameter, from a liquid medium are known. U.S. Pat. No. 4,279,752 describes microporous polyvinyl alcohol membranes prepared by forming membranes containing silica and then extracting the silica. U.S. Pat. No. 4,073,733 discloses porous polyvinyl alcohol membranes prepared by dissolving polyalkylene glycol in polyvinyl alcohol and coagulating the polymer from the solution.
Crosslinked water-soluble polymers are used as an ultrathin solute barrier in a composite membrane disclosed in U.S. Pat. No. 4,559,139. These membranes are useful for separation of potable water via reverse osmosis from brackish water.
U.S. Pat. No. 3,852,388 describes a variety of techniques for making microporous membranes. Phase inversion is a particularly popular technique for making such membranes, but the practical lower limit for the molecular weight discrimination of such membranes is generally 500 or greater.
Japanese Kokai No. 54-118,697 describes a method for making a porous membrane useful to treat body fluids. In this method, a polyvinyl alcohol hollow fiber is coagulated in a caustic solution and then treated with glutaric aldehyde and sulfuric acid to effect crosslinking of the polyvinyl alcohol.
Membranes have been proposed and studied for use in the treatment and reprocessing of industrial by-products. One of the deficiencies of prior art ultrafiltration membranes are that the commercially available polyamide and polyester membranes are not durable in high pH applications. Membranes having improved chemical stability and separation characteristics are actively being sought for use in membrane processes for treating liquids.